So called TTL (through the lens) phase-difference detection-type autofocus is employed widely in film-type and digital single lens reflex cameras. In this type of autofocus system, a light beam that has passed through an image sensing lens is split by a beam splitter, the optical axes of the split beams are shifted relative to each other, images are formed on a focus-state sensor by two image forming lenses, an amount of defocus is calculated from the displacement between the two images and the image sensing lens is driven in accordance with the defocus amount to achieve the in-focus state.
In addition, so-called contrast-detection autofocus is employed widely in video cameras and the like. With this autofocus system, high-frequency components in the image of a subject formed on an image sensing device are extracted and the position at which these high-frequency components reach their highest while the image sensing lens is being driven is adopted as the in-focus position.
A comparison of these two autofocus systems reveals that with the phase-difference detection method, the defocus amount can be sensed directly from the displacement between two images and focusing can be achieved by a single focus-state detecting operation. An advantage, therefore, is that the autofocus (AF) operation can be performed very quickly. On the other hand, the beam splitter for splitting the entrant light beam, the AF image-forming optical system and the focus-state sensor must be provided besides the image-sensing optical system. A drawback, therefore, is higher cost.
With the contrast-detection method, however, it is unnecessary to provide a special AF detection system and, hence, there is an advantage in terms of cost. In addition, since discrimination of the in-focus state can be achieved directly in the image-sensing plane, a further advantage is that focusing precision is excellent. On the other hand, a defocus amount cannot be obtained directly, as it can with the phase-difference detecting method, and it is necessary to find the in-focus position by repeating the operation of detecting the high-frequency components of the captured image while moving the image sensing lens in small increments. In comparison with the phase-difference detecting method, therefore, focusing speed is, in general, very slow.
For these reasons, the TTL phase-difference detecting method has been widely adopted in still-image single-lens reflex cameras that place prime importance on quick focusing. Nevertheless, owing to the fact that the image sensing system and focus-state detecting system are different, the following problem arises in addition to the problem of higher cost:
In the case of ordinary silver-halide emulsion film, the spectral sensitivity characteristic of the image sensing system usually exhibits the greatest sensitivity to light on the order of 400 to 650 nm in order to provide a color reproducibility that conforms to the characteristic of the human eye. On the other hand, a silicon photodiode that performs a photoelectric conversion in an image sensing device such as a CMOS sensor generally has a sensitivity peak on the order to 800 nm. Although it possesses a sensitivity up to 1100 nm on the long-wavelength side, sensitivity is sacrificed and light of a wavelength outside the above-mentioned frequency range is cut by a filter or the like in order to emphasize color reproducibility.
A photoelectric converter serving as a sensor used for autofocusing similarly has a sensitivity up to 1100 nm. However, in a case where focusing is performed under low luminance, or in a case where focusing cannot be performed under low luminance, light from a near-infrared (on the order of 700 nm) light-emitting diode illuminates the subject from the camera and therefore the sensor has a sensitivity up to a wavelength region that is 100 nm longer than that of the image sensing system.
FIG. 8A is a graph illustrating the spectral sensitivity of an image sensing device as well as the spectral characteristics of light sources and of auxiliary light. The abscissa is a plot of the wavelength of light and the ordinate a plot of relative energy. In FIG. 8A, B, G, and R represent the spectral sensitivities of blue, green and red pixels, respectively, of a primary-color image sensing device, and F, L and A indicate the spectral characteristics of a fluorescent lamp, floodlight lamp and auxiliary light from the above-mentioned light-emitting diode or the like, respectively. Further, FIG. 8B is a graph illustrating relative displacement of the focus position, which is due to chromatic aberration of the image sensing lens, versus the wavelength of light.
It will be understood from FIG. 8A that light from the fluorescent lamp contains almost no wavelength components longer than 620 nm, whereas the relative energy of light from the floodlight lamp increases as wavelength becomes longer.
On the other hand, it will be understood from curve C in FIG. 8B that the amount of displacement of the focus position changes in dependence upon wavelength and increases in a direction in which the focal length elongates as the wavelength becomes longer.
Accordingly, in the case where the light source that illuminates a subject is a fluorescent lamp having few long wavelength components, a point in the vicinity of 545 nm, which is the emission-line peak, is the center of the spectral distribution of the light source. As illustrated in FIG. 8B, therefore, the focal length of the lens is a direction that shortens. In the case of a floodlight lamp having many long wavelength components, the longer the wavelength component, the higher the energy and, hence, the focal length of the lens is a direction that elongates. Consequently, a problem which arises is that even if the subject is located at the same position, the image will not be in focus on the side of the image sensing plane.
In order to deal with this problem in which the focus position of an image sensing system is shifted owing to the spectral characteristics of a light source, the specifications of Japanese Patent Publication No. 1-45883 and Japanese Patent Application Laid-Open No. 2000-275512 disclose a camera in which the focus position is corrected in accordance with the type of light source.
The inventions described in these specifications disclose a method of discriminating the type of light source by comparing the outputs of two types of sensors having different spectral sensitivities, and correcting the focus position (i.e., the position of the focusing lens) in accordance with the type of light source discriminated, thereby correcting for a shift in focus ascribable to the spectral characteristics of the light source.
However, with the autofocusing cameras disclosed in the specifications of Japanese Patent Publication No. 1-45883 and of the Japanese Patent Application Laid-Open No. 2000-275512, the amount of correction of the focus position with respect to chromatic aberration is handled as a fixed value. If this is applied directly to an autofocusing camera of the type having interchangeable lenses, the amount of focus correction will be too much or too little in cases where chromatic aberration differs owing to a difference between one lens and another.